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    Re: Sextant vs. Digital Camera
    From: Greg Rudzinski
    Date: 2008 Aug 8, 17:21 -0700

    George,
    
         Very good observations of my post. I did a high altitude check
    using the Cassens and plath with a 4 power scope and had everything
    within a mile and a half with an extreme difference of half a minute.
    So yes there was refraction but that should be expected at very low
    altitude. I did not adjust for flatness so there is another small
    factor. I do have some pictures and I will let you be the judge. I
    believe the pixels are to the nearest pixel which implies a half pixel
    potential error. Keep in mind that all of this is done with a $125
    camera.
    
    On Aug 8, 4:09�pm, "George Huxtable" 
    wrote:
    > Thanks to Greg Rudzinski for posting his interleaved sextant and camera
    >  observations of a setting sun; just the thing we need to assess the accuracy
    > of one against the other.
    >
    > His stated conclusion was this-
    >
    > "CONCLUSION
    >
    > The results demonstrate the ability of a 7 mega pixel camera at optical zoom
    > 4 power �to accurately function �as a sextant "
    >
    > However, I think we need to examine the details first, before agreeing with
    > that conclusion.
    >
    > Because this also is stated-
    >
    > "Sextant observations �observed with CASSENS & PLATH using sight tube and
    > whole horizon mirror."
    >
    > And when we compare the results of the camera and a sextant USED WITH A
    > SIGHT TUBE (i.e., without a telescope) we see that the camera gives an
    > average difference between calculated and observed altitudes of 3.5',
    > against 2.1' for that sextant, �the scatter being about the same, in the two
    > cases,. of 2.2' or 2.1' between extreme values. Not a wonderful performance,
    > for an observation made from firm ground in what are described as ideal
    > conditions. Neither were the sextant observations anything to be proud of;
    > but that may be due to the difficuly of working without a telescope,
    > combined with the problems inherent in correcting for refraction at such low
    > altitudes, and perhaps an unusual value (unknown) for local dip.
    >
    > In which case I would rephrase that conclusion to state only that the camera
    > gave a mediocre performance which was significantly worse than that of a
    > sextant without a telescope.
    >
    > I wonder if Greg Rudzinski has retained any of the images from that set; in
    > particular, the image(s) which were used to obtain the Sun diameter, and
    > from that the overall calibration factor. I am interested to see how sharp
    > and well-defined the Sun limbs were, and whether there was any leeway in
    > assessing the Sun's diameter, at 108 pixels, by choosing a different
    > brightness contour for the fit to the disc. After all, changing that contour
    > by just one pixel, from 108 to 107 pixels, would cause a difference of
    > nearly 1% to the calibration factor, and shift the first of the camera
    > observations in the set by more than 3 arc-minutes
    >
    > Finally, I wonder if Greg has made any assessment for non-linearity of the
    > angular scale that was used, and made any correction based on that
    > assessment.
    >
    > George.
    >
    > contact George Huxtable at geo...@huxtable.u-net.com
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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